Port kio_info to WorkerBase

Tested with konqueror info: as well as in khelpcenter, with additional fprintf to see the new plugin is used,

@sitter The old version did an error() and exit() in thhe constructor, when it could not find its needed resources. What would be the solution-by-design for a WorkerBase to handle that? For now I instead cached the result, keep the worker alive and instead emit an error on each request, as I found no other patern in the existing WorkerBase instances.